Building from commandline setting outputpath without trailing backslash creates invalid output

### Describe the bug

If you build from commandline but set the outputpath directly, it causes several XBF files to be output to the wrong location.

### Why is this important?

I spent all day trying to figure out why my build was suddenly broken, only to discover I wasn't adding a `\` to my output path. This problem occurred when moving from 1.6 to 1.7+

### Steps to reproduce the bug

1. Create a new WinUI project
2. Open a VS Command Line and CD to the project foler
3. Run the command `msbuild /t:build OutputPathBug.csproj -p:Platform=x64 -p:RuntimeIdentifier=win-x64 -p:OutputPath=output`
4. Notice you'll get `outputApp.xbf and `outputMainWindow.xbf` in the current folder, and the generated app will fail to launch.
5. Repeat but add a trailing `\` to the outputpath, and things will be ok.

### Actual behavior

Invalid deployment generated

### Expected behavior

It's not required to add a trailing backslash to the output path.

### Screenshots

_No response_

### NuGet package version

1.7.250310001+

### Windows version

_No response_

### Additional context

Regressed in 1.7.250310001. Worked in 1.6.240923002

Start by reproducing the issue with the provided msbuild command in a new WinUI project, then compare the generated output with and without the trailing backslash and against version 1.6. The fix is complete when OutputPath=output places the XBF files correctly and produces a valid deployment without requiring a trailing backslash.

Written by the indexing model from the issue text.